One thing people often overlook is the idea that if the law enforcement infrastructure crumbles with the government, the individual's options for perimeter defense expand greatly. Laws and civilization are what prevent us from putting in punji sticks, shotgun shell traps, homemade mines, etc. around a home/bunker. If there is no more civilization (but not before that point, since you do your family no good if you're in jail for something stupid when TSHTF), these options would be on the table.

I think that a comprehensive plan with perimeter surveillance, alarms, and traps/mines would serve to alert you to the presence of a gang and sufficiently disrupt their attack/thin their numbers/cause general chaos to give the defender the edge.

We need to think outside the box - for an audible alert, there are DIY door alarms that you can put onto a door and frame so that when the door is opened it starts screaming. Place these two contacts together, put a trip wire hooked to one, and when they separate, you have a siren pointing out the location of the breach. Couple this with a solar power source (the yard lights you stake into the ground with the small solar cell on top could be wired to the alarm instead of the light) and you'd have an indefinite lifespan for the device.

Interesting point of view.

As far as tiger pits and such.....I once dug a 4-5' deep hole and burried a garbage can in it to make a french drain of sorts. Took me quite a while and LOTS of energy.